Attempts have been made for over half a century to add value to metallic materials by making pores therein. Use and development of porous metals are expected to occur in an extremely wide range of applications which include implementation as extra-light materials, high-specific-rigidity materials, energy-absorbing materials, vibration-absorbing materials, soundproofing materials, thermal insulation materials, electrode materials, filter materials, biomedical materials, as well as materials for heat exchangers and oilless bearings, for example. The porous metals also have a high potential as promising materials capable of coping with issues concerning environment, energy and human aging.
Among these metals, porous metallic materials having nanometer-scale fine pores with a pore size of less than 1 μm are highly expected to exhibit high functionality which are unobtainable with conventional materials with respect to catalytic properties, electrode characteristics, gas storage characteristics and sensing characteristics, because the porous metallic materials have significantly large specific surface areas as compared to bulk metallic media. Conventional technology for making porous metallic materials includes a foam melting method, a gas expansion method, a precursor method, a self-propagating, high-temperature synthesis method, a painting method and a spacer method. Pores produced by these methods, however, have a pore size of a few tens of micrometers or larger and it is not easy to reduce this pore size. Thus, porous metallic materials having a pore size controlled to a nanometer scale have been produced chiefly by a dealloying method described below.
The dealloying method is a method of manufacturing a porous medium by removing only a phase whose principal constituent is a less noble metal by dissolving the same in an acid or alkali aqueous solution at room temperature from an alloy or a compound which is characterized by having a complex phase whose principal constituents are a combination of less noble and noble metals having negative and positive standard electrode potentials, respectively, the porous medium being a phase whose principal constituent is a remaining noble metal (refer to Patent Document 1, for example).
Also, metallic materials used as biomedical materials attract attention in recent years. Despite the ever advancing tendency toward a super-aging society which comes from development of medical technology, difficulties for daily life caused by deterioration or loss of functions of various organs will become increasingly serious in the future and, thus, reconstruction of such functions will become an extremely important issue in the medical field. Medical devices using ceramics, polymers and metallic materials have been developed to provide promising means for solving the above problem. The medical devices are broadly classified as follows: orthopedic devices such as artificial joints and bone fixation components; devices for cardiovascular surgery and medicine such as implantable artificial hearts and vascular stents; devices for otolaryngological applications such as artificial inner and middle ears; dental devices such as implants and orthodontic wires; and devices for general surgical applications such as catheters and surgical instruments, for example.
Metallic materials are excellent in various properties, such as moldability, hyperelasticity and shape memory performance. Additionally, metallic materials have excellent strength and toughness as compared to ceramics and polymers. For this reason, approximately 80% of implantable devices whose materials can not be replaced by ceramics or polymers have been developed by using metallic materials. For example, SUS316L stainless steel which is austenitic stainless steel, Ti-6Al-4V ELI, cobalt-chromium (Co—Cr) alloy, Ti-6Al-4Nb, nickel-free stainless steels and nitinol (titanium-nickel (Ti—Ni) shape memory alloy of which atomic percentage of nickel is 48.5 to 51.5%) are widely known and used as typical metallic materials for medical devices.
While metallic materials have high effectiveness as used in medical devices from a viewpoint of strength and toughness, these materials are associated with drawbacks to be overcome at the same time. Generally, when placed in contact with a living tissue, a metallic material ionizes and dissolves due to corrosion, potentially exhibiting a risk of toxicity. It is therefore essential for metallic materials used in medical devices to have high corrosion resistance. Nickel, mercury, cobalt, palladium and chrome are examples of typical metallic elements that can be causes of allergies (allergens), among which nickel-induced allergy is particularly serious. As nickel is noticed also as a carcinogen, individual countries have established nickel elution standards to strengthen their preventive measures. Under such circumstances, new alloys to be used in alloy design for devices using medical metallic materials are being developed primarily on condition that these devices should not contain nickel. A current situation, however, is that this restriction on usable components poses a significant obstacle to alloy development.
A known method for solving the aforementioned problem is a surface reforming method in which nickel is caused to dissolve from the surface of a nickel-containing alloy by using any of various kinds of electrolytic solutions, thereby forming a film containing titanium oxide as a principal constituent with a reduced nickel concentration and suppressing the elution of nickel ions into a living body (refer to Patent Document 2 or 3, for example).